Source : Webster's Revised Unabridged Dictionary (1913)

Hearthstone \Hearth"stone`\, n.
   Stone forming the hearth; hence, the fireside; home.

         Chords of memory, stretching from every battlefield and
         patriot grave to every living heart and hearthstone.
                                                  --A. Lincoln.

Source : WordNet®

hearthstone
     n : a stone that forms a hearth